Cache smartspace timer creation time

Due to a limitation on the smartspace, the creation time passed from the
targets are not reliable. This is a workaround in that SystemUI uses the
timestamp for when a timer first appears for ranking in the Glanceable
Hub.

Test: atest CommunalSmartspaceRepositoryImplTest
Bug: 322869039
Bug: 318535930
Flag: com.android.systemui.communal_hub
Change-Id: I9498dc7d712a7cb72e3ff4b345293638a3eece46